Eli Lilly doesn't expect to see generic competition to its second best-selling drug Cialis (tadalafil) until after September 2018, announcing Wednesday it had entered into a settlement agreement with generic drugmakers to resolve pending patent litigation over the erectile dysfunction treatment.

Lilly had been battling generic companies in federal district court over a patent covering Cialis' unit dose, which was previously due to expire in April 2020. Under the settlement agreement, Cialis exclusivity will end at the earliest on September 27, 2018.

That date is nearly a year after Lilly expects to lose patent protection on other compound and use patents which cover the drug through November 2017. In a statement, Lilly's general counsel Michael Harrington said the agreement provides Lilly with more certainty around its U.S. exclusivity.

With the agreement, Lilly extends the runway for Cialis, staving off a more immediate blow to revenues. That generic threat still looms in the near-future, however, and will post a challenge to Lilly's financial growth.

The Indianapolis drugmaker has committed to hitting 5% annual revenue growth through 2020, betting a wave of new drugs will help see it through another rough patch of patent expiries.

Copycat versions of Eli Lilly's ADHD drug Strattera (atomoxetine) launched earlier this year, and the company anticipates losing exclusivity for its blood-thinner Effient (prasugrel) in October. A recent legal victory in the U.K.'s top court, however, should help Lilly defend its cancer drug Alimta (pemetrexed), which also faces inbound rivals.

Cialis is Lilly's second best-selling product and accounted for nearly 12% of total revenues last year with $2.47 billion in sales. In the U.S., recent growth has come from higher realized prices as the pharma anticipates the looming patent cliff.

"We expect that the entry of generic competition following the loss of exclusivity will cause a rapid and severe decline in revenue," the company stated in its year-end filing with the Securities and Exchange Commission. Such a steep decline will be a heavy blow for Lilly's bottom line, even if it comes a year later than previously expected.

Lilly has weathered major patent losses before. In 2013, sales of its antidepressent and pain drug Cymbalta (duloxetine) topped $5 billion — only to collapse down to $1.6 billion the next year after its patent protection expired.

This time, though, Lilly hopes a group of newly approved and launched drugs — headlined by Trulicity (dulaglutide) and Taltz (ixekizumab) — can keep revenue growth on track.

"With regards to learnings from our YZ experience, the biggest difference is the fact that with this next wave of patent expiries we are able to offset that with new product launches," said Derica Rice, Lilly's CFO, on a recent earnings call, referring to a period of time between 2011 and 2014.

That narrative took a hit, though, when Lilly failed to win U.S. approval for baricitinib, a closely watched drug for rheumatoid arthritis.